A variety of catalyst compositions containing single site catalyst precursors have been shown to be highly useful in the preparation of polyolefins, producing relatively homogeneous copolymers at good polymerization rates and allowing one to tailor closely the final properties of the polymer produced. In contrast to traditional Ziegler-Natta catalyst compositions, single site catalyst compositions comprise catalytic compounds in which each catalyst composition molecule contains one or only a few polymerization sites. Metallocenes are the most well known type of single site catalyst precursor, and are organometallic coordination complexes containing one or more pi-bonded moieties (i.e., cycloalkadienyl groups) in association with a metal atom from Groups IIIB to VIII or the Lanthanide series of the Periodic Table of Elements.
It is known in the art that the presence of olefin comonomers during ethylene copolymerization sometimes increases the activity of the catalyst composition being used. This is often referred to as the "comonomer effect." The comonomer effect has been associated with catalyst composition containing both Ziegler-Natta catalysts and metallocenes. See for example, Karol et al., J. Polymer Sci. Chem., Vol, 31, p. 2541 (1993); Koivumaki et al., Macromolecules, Vol. 26, No. 21, p. 5535 (1993); Herfert et al., Makromol. Chem., Vol. 194, p. 3167 (1993), and Tait et al., Catalyst Design for Tailor-Made Polyolefins, Eds., Soga and Merano, Kodanska, New York, p.55 (1972).
In addition, it is known that both Ziegler-Natta catalyst compositions and metallocene catalyst compositions may be prepolymerized by an olefin prior to introduction into a polymerization reactor. For example, U.S. Pat. No. 4,767,735 to Ewen et al. discloses the prepolymerization of a Ziegler-type catalyst composition for use in olefin polymerization. Prepolymerization is accomplished by adding an olefin monomer to a fluid carrier stream containing a Ziegler catalystc composition, and then passing the monomer/Ziegler catalyst/carrier stream through an elongated tubular reactor at a flow rate sufficient to provide a residence time in the tubular reactor of less than one minute and under conditions to prepolymerize the catalyst composition.
U.S. Pat. No. 4,665,047 relates to a zirconocene/aluminoxane catalyst composition stabilized against aging prior to use by adding to the catalyst composition during or after its preparation at least one mole of a stabilizing olefin of the formula R.sub.1 R.sub.2 R.sub.3 CCH.sub.2 .dbd.CH.sub.2, wherein R.sub.1, R.sub.2 and R.sub.3 are C.sub.1 -C.sub.20 alkyl moieties, per mole of zirconocene. Similarly, British Patent Application No. 1 300 734 describes a titanium trichloride/aluminum trialkyl catalyst composition stabilized by adding thereto during or after its preparation an olefin.
Although catalyst compositions containing single site catalyst precursors have good activity, it is often desirable to be able to boost their activities even further. To this end, applicants have discovered that when a single site catalyst precursor is contacted outside of the polymerization reactor with an activating cocatalyst before, during, or after contacting the single site catalyst precursor with a weakly coordinating electron donor that does not substantially polymerize during either contacting, an activated catalyst composition is formed that has greatly increased activity relative to a similar activated catalyst composition prepared by contacting the same catalyst precursor and the same activating cocatalyst but without contacting the weakly coordinating electron donor.